BENGHAZI, Libya, June 15 (Reuters) - A renegade Libyan general launched an offensive on Sunday against Islamist militants in the eastern city of Benghazi, prompting dozens of families to flee the port in the latest bout of turmoil to hit the North African oil-producing nation.

Libyan authorities are struggling to restore order across the vast desert nation ahead of a June 25 parliamentary election. The situation remains especially chaotic in Benghazi, Libya's second largest city and cradle of the NATO-backed uprising that toppled Muammar Gaddafi three years ago.

Retired general Khalifa Haftar has declared war against militants in Benghazi and several army units have joined him, though the Tripoli government says he has no authority to act.

Residents were seen packing belongings into cars and heading out of the area to escape the fighting.

Hejazi said Haftar had warned the Islamists against shipping in arms via the commercial port of Derna, east of Benghazi.

Derna is a hotspot for Ansar al-Sharia and other militant groups amid Libya's persistent power vacuum. The United States designates Ansar al-Sharia a terrorist organisation.

Haftar was once close to Gaddafi but fell out with him and then played a role in the 2011 uprising. In February, he stirred rumours of a coup by appearing in military uniform to call for a presidential committee to be formed to govern till an election.

Libya's government and parliament are paralysed by divisions between Islamists and more moderate forces as well as competing tribes and regions. (Writing by Ulf Laessing; Editing by Gareth Jones)